Haloperidol also appears to be a high-affinity ligand at sigma binding sites.
In fact, the term "haloperidol-sensitive sigma receptor" has recently been
coined to describe a subclass of sigma sites that appear to be distinct from
other PCP- or opiate-associated sites. I have not been following the sigma
story very closely, and I'm not sure whether the physiological and functional
consequences of haloperidol's actions at the sigma site have been determined.
Perhaps you could run a Medline search on Sigma receptors and haloperidol to
get the latest info on this topic